A cathode is an electrode through which electric current flows out of a polarized electrical device. X-rays are produced by bombarding a target (the cathode), which is usually made of tungsten.

The anode is the positive terminal of an x-ray tube, usually consisting of a tungsten block embedded in a copper stem. Electrons flow from the cathode toward the anode and the anode emits x-rays from the focal spot.

The cation is a positive charged ion. Cations are attracted from the cathode.

The filament is the source of electrons (cathode) in x-ray tubes. A thin wire (0.1- 0.5 mm, usually tungsten) emits electrons due to thermionic emission, operating in a vacuum and energized with electric current.
A CT tube utilizes a larger filament with larger size of the effective focal spot than a conventional x-ray tube.

The focal spot is the point where the electron beam impinges on the tube anode and from which x-rays are emitted. The size of the focal spot is determined by the size of the filament and cathode, which is determined by the manufacturer.
Most x-ray tubes have more than one focal spot size. The size of the focal spot has influence on spatial resolution. The smaller the focal spot, the better the limiting spatial resolution of the x-ray system, especially in magnification mammography. The use of a small focal spot concentrates heat onto a smaller portion of the anode therefore, more heat is generated and a longer cooling time is necessary.